a) Field of the Invention
The present invention relates to a cored multi-shell synthetic resin emulsion particle useful as an additive to a paper coating agent utilized for common coated papers, paper boards, lightweight coated papers, ultra-lightweight coated papers, art papers and cast coated papers, to a paint applied to wood, external and internal walls, and to a coating agent used for thermal recording materials of facsimile papers, thermal labels and other thermal recording papers, comprising a core particle and a void layer formed by drying the exterior of the core particles; and a resin composition for paper coating, a resin composition for paint and a thermal recording material comprising said particle.
b) Description of the Related Art
In recent years, various high molecular weight polymer particles have been studied as additives to coating agents.
The most commonly used particles are homogeneous and solid, emulsion-polymerized polystyrene particles having a diameter of 0.2 to 0.5 micron (.mu.). For example, Japanese Patent Laid-Open No. 59741/'84 discloses a process wherein an unsaturated carboxylic acid and a vinyl monomer are copolymerized in the presence of an anionic surface active agent and/or a nonionic surface active agent to form a copolymer emulsion in which more than 90% of the particles have a diameter of 0.20 to 0.28.mu.. It is also described therein that the emulsion particles can be used as an organic pigment in applications including paper coating, paints and thermal recording materials. The solid emulsion particles, however, have a common problem that hiding power, brightness and gloss are insufficient when used in an organic pigment and hence no practical advantage is obtained unless they are used in a large amount.
In order to improve hiding power, brightness and gloss of the solid emulsion particles, an organic material which has pores after drying has recently been proposed as a substitute of the uniform and solid particle, for example, in U.S. Pat. No. 4,427,836 and Japanese Patent Laid-Open No. 62510/'86.
The former discloses a process for the preparation of an aqueous dispersion which comprises copolymerizing at least 5% of an unsaturated carboxylic acid to form a core material, adding at least one monoethylenically unsaturated sheath monomer, conducting emulsion polymerization to form a sheath polymer, neutralizing the resulting emulsion particles with an aqueous volatile base to swell the core polymer, thereby forming minute openings in the core by drying the particle.
The latter describes a process for the preparation of synthetic resin emulsion particles having empty pores in the interior by utilizing phase separation between different kinds of polymers in the polymerization reaction and volumetric shrinkage resulting from the polymerization.
The hollow particles produced by the above processes have improvements in hiding power, brightness and gloss as compared to conventional solid particles. The hollow particles, however, have following disadvantages, when applied to uses described below.
(1) The use of hollow particles as an organic pigment for paper coating:
The hollow particles are excellent in any of hiding power, brightness and gloss as compared to conventional solid particles. On the other hand, enhancement of performance in paper coating is more stringently required. Hence the hollow particles are generally used in the same range of amount as that of the solid particles. The procedure is effective to a certain extent for increasing hiding power, brightness and gloss. The procedure, however, decreases the strength of the coated layer and causes problems in printability.
Further, in a calender treatment conducted in order to increase gloss after applying a coating liquid, it is generally carried out to enhance flatness of the coated surface by increasing either or both of temperature and pressure. However, the conventional particles having empty pores only in the interior thereof are liable to collapse due to their empty structure leads to a serious decrease in hiding power and brightness. The decrease in strength in the coated layer and the deformation of the particles caused by heat and pressure also result in problems such as blocking or adhesion of dirt to the calender rolls, which circumstances is a serious disadvantage for practical use.
In order to improve the above disadvantages, counter measures such as change of particle diameter or change of void ratio in the interior of the particles having empty pores can be carried out. Pore size of about 0.22.mu. is needed to exhibit maximum hiding power, brightness an gloss (U.S. Pat. No. 4,427,836). Even though the void ratio in the interior of the particles is changed, preventing generation of blocking in the calender treatment and enhancing the strength of coated layer cannot be achieved.
As mentioned above, it has been difficult to obtain an organic pigment for paper coating using the particles having empty pores.
(2) The use of hollow particles in paints:
The hollow particles are excellent in hiding power and brightness as compared with conventional solid particles. However, as has been pointed out a disadvantage is that the hollow particles have slow emergence of hiding power and brightness in practical coating operation. Since the desired whiteness cannot be obtained immediately after application, workers make; and mistake of performing a recoat in excess and the resulting coated layer becomes thicker than desired or has an irregular thickness.
The phenomenon is also a problem which cannot be solved even though the particle diameter or the void ratio in the interior of the particles is changed as in the case of paper coating.
(3) The case of using for thermal recording materials:
As a result of recent progress in the diversification of information equipment, the thermal recording materials used are also required to improve the properties and defects such as coloring sensitivity, printing visibility, brightness, foreign matter adhesion to a thermal head, and sticking so as to meet high-speed recording and energy reduction in the thermal head and a hot pen.
Japanese Patent Laid-Open No. 86789/'80 has disclosed a method for obtaining a dense and distinct image by forming an intermediate layer containing synthetic resin particulates between a substrate and a thermally color-developing layer. The method, however, is difficult to obtain sensitivity enough to withstand high-speed recording required.
Japanese Patent Laid-Open No. 143683/'84 describes a method for adding styrene-base crosslinked particulates to a thermally color-developing layer in order to prevent foreign matter adhesion, sticking and color development by scratching pressure. With this method, however, it is still impossible to obtain a dense and distinct image and has no practical use.
In order to allow the heat of a thermal head to effectively act on a thermally color-developing layer, a method for obtaining a dense and distinct image by inserting between a substrate and a thermally color-developing layer an intermediate thermal-insulation layer containing thermoplastic particles having one empty pore in the interior of each particle has been disclosed, for example, in Japanese Patent Laid-Open Nos. 117787/'87 and 21180/'88.
However, the particles used in these method slow down the drying speed of the intermediate layer and cause problems such as workability in the successive coating of a thermally color-developing layer. The method does not overcome the problems of foreign matter adhesion on the thermal head and sticking. Further, hiding power and brightness of thermal recording material itself are still insufficient and cannot yet satisfy demand for high performance.
The problems mentioned above cannot be solved even though the diameter and the void ratio of the particle having an empty pore are changed as in the case of using for paints.